True, but as the OP points out, it's a bit of a chicken and egg situation - if you avoid a road because it appears to be jammed up, you won't be in a position to submit that "all-clear" data. This isn't a problem unique to Waze though - the crowd-sourced data used on parts of Google Maps also suffers from this problem - and even with roadside monitoring the data can appear incorrect in certain situations (e.g. a road closure leading to zero traffic along a stretch of monitored road, which then appears to be free flowing...)

The problem is then how long you allow historical data to remain in use before assuming it's no longer valid. Time it out too quickly and you risk routing people straight into a still-present jam, which would be annoying. Time it out too slowly and you risk sending people off down less optimal routes to avoid a now non-existent jam. This is probably less annoying, as unless you know the jam is no longer present then you'd still think Waze was doing its job correctly, although if the route you've been asked to follow is a bit tricky to drive then you might be cursing Waze anyway by the time you've regained your original route...

Or someone breaks down, leaves Waze running and you get routed off the motorway at the previous junction.Yes this has happened to me. Annoying but nice to see that rerouting does in fact work. I realised when I was sent across the bridge over the motorway and kindly cursed my phone.